The invention relates generally to the electrical discharge machining process. Specifically, the invention provides alternative cycles of operation of an EDM power supply in response to the detection of a unique control variable.
Typically, power supplies produce machining pulses which have variable ON times and OFF times. As disclosed in prior art, during the ON time, two discrete pulses may be applied to the machining gap. The first pulse is a low power pulse for creating a low power discharge. Immediately, following each low power pulse, a machining pulse is applied to the gap to generate a machining discharge thereby removing material from the workpiece. The low power pulse is used as a mechanism to supply information to control the subsequent operation of the power supply. This low power pulse should not be confused with a high voltage gap initiation circuit in which a separate high voltage signal is applied to the machining gap to ionize the gap thereby initiating a machining discharge. The main objective of the present disclosure is to apply a low power pulse to the machining gap prior to the application of the high voltage gap initiation signal and subsequent machining pulse. During the low power pulse, the voltage signal across the gap is detected to control the generation of subsequent machining pulses.
The prior art proposes that the current flowing through the gap during a low power discharge be measured. However, current as a measured variable has certain limitations. Because of the inherent stray capacitance in the system which must be charged by the low power pulse, the current signal contains an initial transient or overshoot which makes the detection of a stable discharge current very difficult. When working with very short ON times, the current does not have time to reach a stable state before the low power pulse is terminated. In this situation, the detection of current will provide unreliable information. To overcome this disadvantage, the present disclosure recites the detection of a gap voltage signal in response to low power pulses. The gap voltage signal does not contain a leading edge overshoot and provides similar information as to the general quality of the machining gap.
The present disclosure recites a mode of operation of the power supply in response to the detection of an inferior gap voltage signal produced by a low power pulse which is particularly useful where pitting of the workpiece is a problem.